The demands of mass production of metal parts for appliances, vehicles and machines of all types has driven the technology of powder metallurgy ever since it was discovered that a mixture of the appropriate finely divided metal particles in the form of a powder, mixed with a binder and lubricant, each also in finely divided form, could be compacted and sintered to yield an article of arbitrary shape and size. In the art, “powder metal mixture” refers to a mass of particles each of which is less than 150 μm (micrometers or microns) in average equivalent diameter (“avg. equiv. dia.”), preferably its largest dimension, the metal particles typically being largest, preferably having an avg. equiv. dia. of less than 75 μm, the smallest metal particles having an avg. equiv. dia. of about 25 μm, and the non-metallic additives typically having an avg. equiv. dia. smaller than that of the metal particles, typically less than 50 μm. Depending upon the physical properties demanded of the sintered, or sintered and heat treated, ferrous article, a wide array of powder additives are added. The most common of such additives are a lubricant and/or graphite, followed by metals such as nickel, copper, molybdenum, manganese, chromium, cobalt and/or an organometal or metal compounds such as sulfides, phosphides, and the like which become alloyed with the powder metal when sintered and/or heat treated.
The term “powder metal mixture” in this invention refers to a mixture of ferrous metal particles in which mixture the ferrous metal powder is present in an amount greater than 90 per cent, the remaining ingredients being additives such as a lubricant, optionally a binder which may be the same as the lubricant or different, and alloying ingredients such as graphite and metals, each present in an amount less than 2 per cent by weight (% by wt) of the total mixture including the powder.
The term “lubricant” refers to a powder of particles no dimension of which is more than about 100 μm, and typically having an avg. equiv. dia. in the range from about 5 μm to 25 μm; in this invention the lubricant is modified to consist essentially of a lubricant powder such as is conventionally used to make a compact mass of powder metal, blended with particulate starch having an avg. equiv. diam. length less than 150 μm, preferably less than 50 μm, optionally in combination with no more than an equal amount of micron-sized fragments cellulose fibers, like starch, a polysaccharide, provided that 90% of the fragments have a length less than the avg. equiv. dia. of the metal particles. Modification of a conventionally used powder metal lubricant with starch alone, for convenience and brevity, is referred to as “starchlube”.
Because powder metal particles, like starch particles may be irregular in shape, the equivalent diameter of a particle refers to its diameter had it been a sphere of equal volume.
The cellulose fragments which may be mixed with starch particles are preferably much smaller than 50 μm and in a narrow size range, that is, at least 90% by wt are less than 10 μm in length, and the average length of all fragments is in the range from about 4-5 μm. The fragments are referred to as being “micronized” as they are conveniently obtained by feeding cellulose fibers in short lengths less than about 6.35 mm (0.25″) to a commercially available jet classifying mill or “micronizer” such as a Model 30 Roto-Jet manufactured by Fluid Energy A1-Jet Company. Most preferably the cellulose fibers, such as cotton, hemp, manila, sisal, jute and the like, are first irradiated with enough radiation to alter their surface structure, preferably by exposing the fibers to electron beam radiation to receive a dosage, typically measured as kiloGrays (kGy), in a range equivalent to from about 30 to 100 MegaRads, as described in detail in the aforementioned '712 patent, the disclosure of which is incorporated by reference thereto as if fully set forth herein. Because, like starch particles, cellulose fragments have no notable lubricity they are together referred to as a “non-lubricant”.
The overriding criterion for a practical powder mixture is its homogeneity without which there would be an unacceptable variance in composition of a compacted metal part, not only from part to part, but within a part itself. The term “part” is used interchangeably with the more formal term “article”. Such homogeneity encompasses not only the distribution of particles within a mass of unit volume of powder but the bulk density (measured as “Hall apparent density”) and flow characteristics (measured as “Hall flow rate”) of the powder mixture. The apparent density is the mass of a unit volume of non-compacted powder. Hall apparent density is measured as set forth in ASTM B-212 (Metal Powder Industries Federation “MPIF” test method 04 in “Standard Test Methods for Metal Powders and Powder Metallurgical Products). The flow rate is quantified as the time required for a powder of standard weight to flow through a Hall flow meter. The Hall flow rate is dictated by ASTM B-213 (MPIF test method 03). A variation in bulk density and flow will result in a variation in the “fill”, which is the amount of powder mixture filled in a die cavity before the mixture is compacted, and the dimensions of the compacted part. To a lesser extent, a lack of homogeneity is reflected in variations in green strength of a compacted part particularly in sensitive portions of the molded part, such as the teeth of a gear.
Mainly because adequate green strength is obtained by increasing compacting pressure, green strength as a serious problem attracts attention only in those instances where compaction, or molding pressure is already so high as to shorten the useful life of a die noticeably and/or a worrisome number of compacted parts are damaged when being ejected from the die, or being automatically conveyed to a sintering oven under conditions which cannot preclude the green compacts from being impacted, even if not forcefully.
Green strength is measured as pressure required to break an unsintered compact (a standard rectangular bar) as set forth in ASTM B-312 (MPIF test method 15).
Though the terms “binder” and “lubricant” are used as if to specify different functions in a powder metal blend to be compacted, in practice, the same function may be discharged by a single ingredient, though each function may be to a greater or lesser extent than when discharged by different highly specialized materials. In particular, ethylene-bis-stearamide (“EBS”) is sometimes referred to as a “binder” though it may also function as a lubricant, and metal soaps and waxes are typically referred to as “lubricants” though they may also function as binders. The term “blend” refers to a ferrous metal powder including all ingredients essentially homogeneously dispersed and in condition to be compacted. By “ferrous” metal powder is meant one in which the metal particles contain predominantly the element iron (Fe), typically at least 75% Fe. The binder binds particles of graphite and alloying components to the surface of metal particles. The lubricant reduces friction generated when the powder is subjected to shear, or, stressed; thus, metal powder with lubricant particles flows more easily than without the lubricant; and, a powder mixture with lubricant added to the metal particles may be compacted in a die under pressure and, if the compact has sufficient green strength, ejected from the die with less wear and tear on the die parts.
Typically, the green article is then sintered. The strength of the sintered bar is measured as “transverse rupture strength” (“TRS”) using a standard TRS fixture as described in ASTM B-528 (MPIF test method 41). If the TRS is satisfactory, the tensile strength will generally be satisfactory. Depending upon the composition of the mixture of powder metal and additives from which the article is molded, and its end use, it may be subjected to further processing steps such as sizing/coining, resintering, heat treating, and others.
Recognizing that the component powders of metal, binder and/or lubricant and one or more additional additives differ in size, density and shape, the problem of homogeneity is minimized by choosing particles of comparable size and shape and thoroughly mixing the various particles before using the powder mixture. In this framework, it is evident that any thought of mixing starch particles optionally with an organic fiber of vegetable material with the lubricant and metal particles, no matter how beneficial the non-lubricant might otherwise be, will be quickly dispelled.
Moreover, to date, starch particles have not been used as a lubricant or binder in a powder metal part, because starch, like cellulose and other polysaccharides would not be expected to have any measurable beneficial value for this purpose. Though both starch and cotton have similar chemical formulae, cellulose is a polymer of cellobiose and starch is a polymer of amylose. Each polymer is structurally different and the difference in structures results in greatly differing properties. For example, starches are generally edible by humans, while cellulose is not. Moreover, polycellobiose occurs naturally as fibers, while starch is particulate.
Addition of starch particles in an amount as little as 2% by wt of the total powder mixture, whether of a water-soluble starch or a water-insoluble starch, having an average diameter no smaller than 50 μm, though in the same size range as the avg. equiv. diam. of metal particles used in a compactable powder mixture, results in both unacceptable bulk or apparent density as well as flow characteristics, if the mixture is to be used in the mass production of compacted and sintered parts. The poor physical properties of a powder metal mixture containing starch particles 50 μm and larger is attributable to the volume the particles occupy and the irregularity of their individual shapes. Addition of starch particles smaller than 50 μm, referred to as “micronized starch”, optionally in combination with micronized cotton fibers in a total amount as little as 2% by wt, no matter what their respective proportions, produces the same effects as with starch alone.
On the other hand, it is well recognized that the increased surface area contributed by the smaller particles in a powder mixture relative to the area contributed by the larger ones, impairs the “flowability” or flow characteristics of the powder mixture, resulting in a longer time required to fill a die and additional risk of non-homogeneity in compacted parts. Though such flowability is not a problem when compacting a dozen parts or so in a laboratory, the problem may be critical in a production facility where the number of parts which can be produced per unit time is a deciding factor.
Another serious problem which has loomed large in recent years is the extent of “dusting”, particularly of graphite, and the harmful side effects of vaporizing zinc stearate, commonly used as a lubricant, during sintering. To cope with the latter problem, particularly having to clean sintering furnaces and their flues, the art is proliferated with disclosures of numerous other lubricants including waxes and metal soaps. To minimize or eliminate the use of zinc stearate, polyethylene oxide in combination with an oligomeric amide is disclosed in U.S. Pat. No. 6,511,945; and EBS or a polycarboxylic acid amide wax is used as a binder, but making a homogeneous powder mixture typically requires heating the wax to distribute it uniformly as a coating on the metal particles, as disclosed in U.S. Pat. No. 5,480,469 to Storstrom et al. and U.S. Pat. No 6,573,225 to Vidarsson et al. respectively. To improve lubrication, U.S. Pat. No. 6,413,919 uses a combination of two well-known lubricants, each effective in its own right, one a fatty acid mono- or bis-amide, e.g. EBS, the other a metal soap, e.g. zinc stearate, and relies upon processing the mixture to form a core of one lubricant coated with the other.